An apparatus of the kind described above is disclosed in U.S. Pat. No. 4,808,922.
The known arrangement of magnetic field sources comprises four or a higher even multiple of magnets or magnetic field coils which are accommodated in a housing on both sides of the rotating end faces of a rotatable cuvette arrangement.
Each of the magnetic field sources is surrounded by a coil which can receive the changes of the magnetic field produced by the paramagnetic material and transmit the changes to an evaluation device. The direction of the magnetic fields of the magnetic field sources and measuring coils is matched alternately to each other so that disturbances caused by stray fields or by microphony are compensated for. An especially precise alignment of the magnetic field sources and measuring field coils lying opposite each other is necessary because the measuring result is essentially dependent upon the penetration of the measuring cuvette by the magnetic force lines. The smallest deviations become noticeable as a different flux density which leads to undesired fluctuations of the measured value. Furthermore, it is necessary to provide for the rotation of the measuring cuvette so that it is free of vibration to the extent that a complex journalling of the drive shaft is required which provides a precise fit.
During rotation of the cuvette, the field lines of the magnetic field sources penetrate the individual cuvette sections in different proportions. The magnetic field sources of constant flux thereby penetrate the individual measuring sections of the cuvette arrangement with a different magnetic potential during a rotational movement. This leads to the condition that the measuring signal increases from a low value to a maximum value as soon as the maximum magnetic potential is reached when the measuring cuvette enters into the magnetic field and decreases in a corresponding manner when the measuring cuvette leaves the magnetic field. These signal fluctuations require additional complexity with respect to the processing of the measured value insofar as fluctuations in the measured value are to be eliminated. Stray fields arise because the drive shaft passes through the arrangement of the magnetic field sources and these stray fields are usually caused by the metal drive shaft. The stray fields rotate with the cuvette and likewise falsify the measuring result.